YRestriction.cs

public class YRestriction :  MonoBehaviour{

public float padding = 1f;
private float ymin;
private float ymax;

//here we set our top and bottom barriers
void Start(){
    float distance = transform.position.z - Camera.main.transform.position.z; //not really necessary in 2D, but good habbit
    //take the bottom-middle, to-middle point of our camera and get it's world coordinates
    Vector3 buttomBarrier = Camera.main.ViewportToWorldPoint(new Vector3(0.5, 0, distance));
    Vector3 topBarrier = Camera.main.ViewportToWorldPoint(new Vector3(0.5, 0, distance));
    ymax = bottomBarrier.y + padding;
    ymin = topBarrier.y - padding;
  }

void Update(){
    float newY = Mathf.Clamp(transform.position.y, ymin, ymax);
    transform.position = new Vector3(newy, transform.position.y, transform.position.z);
  }


}